The present invention relates to a magnetic chuck controller and more particularly to such a controller in which current sensing is employed to determine the dissipation of a previously applied current in the chuck winding.
Magnetic chucks are widely utilized in the machine tool industry for holding a work piece which is to be machined or ground. The magnetic chuck is essentially an electro-magnet which is energized to retain the work piece. However, to release the work piece it is typically necessary to provide a demagnetizing sequence, i.e. to reduce the residual magnetism in the chuck and the work piece in order for the work piece to be removed. The demagnetizing sequence typically comprise the application of a succession of successively reducing current levels in successive phases, the polarity of the current being reversed in successive phases. Since the chuck, together with the work piece, constitutes a highly inductive load, the time periods required are relatively long as compared with the typical period of supply line alternating current. Further, in order to demagnetize effectively, time must be allowed for the magnetic flux to penetrate the work piece against the counteracting forces of eddy currents, etc.
In order to shorten the demagnetizing cycle as much as possible, it has previously been proposed to utilize current sensing during the build up of current during each phase in the demagnetizing cycle. Such proposals are for example contained in the Littwin U.S. Pat. No. 3,401,313 and the Wilterdink U.S. Pat. No. 4,402,032. It has been found, however, that this mode of speeding the demagnetizing cycle can reduce the effectiveness of the demagnetizing since it makes no allowance for the time required for the magnetic flex to penetrate the work piece to maximum depth. It has also been proposed to shorten the time required to dissipate a current previously induced in the chuck winding by shunting or "crowbarring" the winding following a period of energization thereof.
While the application of a reverse voltage through a second set of triggerable semiconductor current switching devices would, in theory, more quickly reduce the current flowing, as a practical matter such a technique may induce failures of the semiconductor devices since triggering the second set of devices may produce an effective short across the a.c. supply mains if the first set has not commutated. As is understood by those skilled in the art, it is the nature of an inductive load to freewheel through a triggerable current switching device and keep it forward biased and conducting even though it is not triggered. Thus, though such an arrangement has previously been proposed, i.e. in the Wilterdink patent identified above, there are concomitant problems.
Among the several objects of the present invention may be noted the provision of a novel magnetic chuck controller; the provision of such a controller which facilitates rapid ard complete demagnetization of a chuck and its work piece, notwithstanding variations in the size and magnetic characteristics of the work piece; the provision of such a controller which is highly reliable nothwithstanding the inductive character of the magnetic chuck; the provision of such a controller which is highly flexible in operation; and the provision of such a controller which is of relatively simple and inexpensive construction. Other objects and features will be in part apparent and in part pointed out hereinafter.